The invention relates to anchor shell assemblies for use in conjunction with mine roof bolts and rock bolts.
A variety of mine roof bolt assemblies are known in the art. They usually include a bolt of from three to six feet in length, a roof plate or support plate through which the roof bolt extends, and an expansion shell assembly threaded onto a threaded upper end of the roof bolt. A mine roof bolthole is drilled, usually perpendicular to the surface of the mine roof, and the expansion shell assembly installed on a roof bolt is inserted into the roof bolt hole, such that the support plate abuts the mine roof. The roof bolt then is tightened, causing the expansion shell assembly to expand, thereby anchoring the entire assembly, including the roof bolt, to the mine roof strata and forcing the support plate upwardly against the mine roof. Mine roof strata is composed of various layers of different types of rock having varying strength characteristics. It has been long known that a plurality of spaced mine roof bolts installed in mine roof tends to bind the various layers of mine roof strata together and prevents "slippage" therebetween and greatly increases the strength of the laminated strata structure, thereby preventing "cave-ins" of the mine roof.
However, in mine roof strata that is in the range from "medium hard" to "soft", conventional anchor shells, such as those shown in U.S. Pat. Nos. 691,921 and 3,139,730, are not always effective, and cave-ins occasionally occur even when such anchor shells have been utilized in conjunction with mine roof bolts to attempt to bind the mine roof strata together. Such conventional anchor shells generally exert compressive forces radially outward from the roof bolt shank as the expansion "wings" thereof swivel radially outward from the base of the anchor shell. A common limitation of conventional anchor shells is that when the conventional anchor shell is attached to the threaded end of the roof bolt, the threaded end is permitted to extend too close to the bail (bight), so that as the wedge nut advances down the bolt stem the end of the bolt stem contacts the bail before the wedge nut has caused the wings to expand sufficiently to provide anchorage against the bolt hole wall.
Another common limitation of conventional bolt anchors is that in soft strata, there is insufficient expansion (limited range of outward movement) to penetrate deeply enough to provide anchorage. In this situation, the bolt simply pulls the anchor downward until the wedge nut reaches the end of the threads on the bolt (standard thread length on the bolt is about 6 inches) and no anchorage is provided. For example, in the device shown in U.S. Pat. No. 3,022,700, the anchor shell assembly includes a bight portion that is forced upward by the roof bolt as it is tightened, pulling the outer portion of the anchor shell upward, while the threads of the roof bolt force a wedge nut between the outer portions of the anchor shell, causing them to expand and engage the walls of the roof bolt hole. If the material surrounding the roof bolt hole is so hard that little expansion of the outer portion of the anchor shell can occur, the torque required to further tighten the roof bolt may increase to a level greater than the specified torque limit before the base plate is drawn against the mine roof surrounding the roof bolt hole, in which case that roof bolt performs no useful function.
Most conventional anchor shells expand outward radially exerting a horizontal (longitudinal) compressive force against the bolt hole wall. Some shells have a slight outward pivot motion from the lower end of the shell, but lack a large amount of pivotal motion outward, and also lack pivotal motion from both ends of the shell. However, if an anchor shell can allow a large degree of pivotal action of the expansion wings, there is a resulting capacity for greater penetration depths of the wings into the roof strata. This can be advantageous in soft to medium hard strata.
Accordingly, it is an object of the invention to provide an improved expandable shell assembly for a roof bolt or the like that can be adequately anchored into either hard or soft mine roof strata surrounding a roof bolt hole and adequately draw a roof plate against the roof in either case.
It is another object of the invention to provide an expandable anchor shell assembly that digs into relatively soft mine roof strata to provide an adequate anchor for a mine roof bolt and also tightly engages hard mine roof strata surrounding a roof bolt hole without greatly increasing the torque required to further tighten the roof bolt.
It is another object of the invention to provide an improved expandable anchor shell assembly that allows adequate forcing of a base plate against the portion of a mine roof surrounding a roof bolt hole, wherein the anchor shell assembly is anchored in either relatively soft or relatively hard mine roof strata.